I would argue that asking people do do 100 miles in a day with luggage, then a crappy night sleep on a ferry and another 60 the next day might be a bit much for a 'newer rider'. You want to make it fun, not a chore.
I make it 120 miles on a bike between Manchester and Hull with two large bumpy bits in the middle.
I'd say that is way beyond the abilities of a casual rider, loaded with luggage, to do in a single day
That would be 14-16 hours of riding time, even a decent rider would not be under 12 hrs.
In reality the Manchester Hull section is three days riding, 30 miles first day, and under 50 miles for each of the other days, followed by a night on the ferry and then a 50 mile ride between Hook of Holland to Amsterdam as the 'big' day

) and the only real climb of any note on the whole route is a local one; I'll work on getting them all over it at least once prior to the journey... if necessary I'll put all the luggage in the car and get SWMBO to meet us at the top so they can do it unladen 